For over two decades, the Environmental Protection Agency ("EPA") has considered four exposure pathways in determining whether to list contaminated sites on the Superfund National Priorities List ("NPL"): groundwater, surface water, soil and air. In a Federal Register notice published January 31, 2011, EPA solicited public comment on the potential addition of a fifth pathway: soil vapor intrusion.

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The migration of vapors from subsurface contamination into overlying buildings, known as vapor intrusion, is a growing concern for federal and state environmental regulators. Vapor intrusion is most common at sites with elevated levels of volatile organic compounds — including chlorinated solvents and sometimes gasoline — which enter indoor air through openings around sewer lines, cracks in a building's foundation or basement, or other preferential pathways.

Under the federal Superfund law, EPA screens contaminated sites for listing on the NPL through its Hazard Ranking System, assigning each site a score based upon its perceived threat to human health and the environment. The risk of vapor intrusion, however, does not currently factor into this determination. A May 2010 Government Accountability Office ("GAO") report found that, given EPA's inability to designate NPL sites on the basis of vapor intrusion, "States may be left to remediate those sites without federal assistance, and given states' constrained budgets, some states may not have the ability to clean up these sites on their own."

From now through April 16, 2011, EPA will be collecting public comment on the potential revision of the Hazard Ranking System ("HRS") to account for vapor intrusion. It plans to hold three public listening sessions on the topic. While it has not proposed specific regulatory changes at this point, the Agency "will consider the information gathered from this Notice, listening sessions, and other sources before making a decision to issue a proposed rulemaking to add subsurface contaminant intrusion to the HRS."